This invention relates to a process for the preparation of organic polyisocyanates in the presence of solvents in which the solvent is reused.
Organic polyisocyanates are produced on a large industrial scale by phosgenation of the corresponding primary polyamines in the presence of inert organic solvents such as chlorobenzene or orthodichlorobenzene (see e.g. Ullmanns Encyclopadie der technischen Chemie, 4th Edition (1977), Volume 13, pages 350 et seq).
In the preparation of the industrially important polyisocyanates, particularly in the preparation of hexamethylene diisocyanates, tolylene diisocyanates or polyisocyanates of the diphenyl methane series by phosgenation of the corresponding di-and polyamines, traces of byproducts containing isocyanate groups are invariably formed (e.g., 6-chloro-hexylisocyanate in the preparation of hexamethylene diisocyanate, tolyl isocyanate in the preparation of tolylene diisocyanates and phenyl isocyanate in the preparation of polyisocyanates of the diphenyl methane series by the phosgenation of aniline/formaldehyde condensates). Such unwanted isocyanate compounds seriously impair the quality of the desired end products (polyisocyanates). It has therefore been attempted to remove these impurities from the polyisocyanate by distillation together with the solvent after the phosgenation reaction and subsequently free the solvent from these impurities by an elaborate column distillation. The solvent can then be reused. This purification of the solvent by distillation requires considerable consumption of energy and expenditure in apparatus, and particular difficulties are encountered when the compounds have boiling points close to those of the solvents used.
German Offenlegungsschrifrt No. 3,129,270 describes a process for the preparation of polyisocyanates in the presence of solvents, in which the solvent is freed from traces of compounds containing isocyanate groups before it is reused. The solvent is treated with compounds containing isocyanate reactive hydrogen atoms, such as alcohols or amines, to convert the readily volatile isocyanates into reaction products containing urethane or urea groups. The treated solvent is then separated from these reaction products by distillation. Even though these reaction products (which have much higher boiling points than the isocyanate traces) are much more easily separated by distillation, removal of these by-products necessitates distillation of the whole quantity of solvent required for the preparation of the polyamine solution. This entails a high expenditure of energy due to the large quantity of solvent required.
If the ureas or urethanes formed in the process described in German Offenlegungsschrift No. 3,129,270 are not removed by distillation, they enter the phosgenation process when the solvent is subsequently reused and may undergo numerous further reactions with phosgene and with the newly formed isocyanates. The quality of the polyisocyanates and the yield of the diisocyanate which is to be distilled are thereby diminished.